On August 19, 2025, London Zoo was abuzz with activity—but not from the usual crowds or school groups. Instead, it was the animals themselves who took center stage, lining up (sometimes reluctantly, sometimes with a little snack-based encouragement) for the zoo’s Annual Weigh In. This much-anticipated event saw more than 400 creatures—from towering Galapagos tortoises to diminutive Darwin’s frogs—step, crawl, or hop onto the scales as keepers recorded their vital statistics.
The tradition of weighing and measuring animals at London Zoo is hardly new, but it’s far from routine. As Daniel Simmonds, the zoo’s animal operations manager, explained to LondonWorld, “The London Zoo Annual Weigh In is not just about numbers on a scale; regular weigh-ins help zookeepers track the health of our animals—including the capybaras, tortoises, and penguins—and genuinely help us manage the care of these animals, who play a vital role in our global conservation efforts.”
The process is as much about creativity as it is about science. Some animals, like Priscilla—a stately 30-year-old giant Galapagos tortoise—are lured onto the scales with irresistible treats. Priscilla, for her part, was coaxed onto a board scale with the promise of a crisp lettuce snack, which she munched contentedly as keepers recorded her weight. Others, such as the sprightly capybaras Gizmo and Kiwi, seemed almost eager to participate, hopping onto the scales with little fuss. For more skittish or suspicious residents, keepers have devised clever tricks, like hiding scales in doorways or camouflaging them in familiar surroundings, to ensure everyone gets their turn.
It’s not just the large or charismatic animals that get the spotlight. The zoo’s tiniest occupants, like the Darwin’s frogs—each measuring a mere two centimeters in length and tipping the scales at under two grams—are just as carefully weighed and measured. Watching a keeper gently place such a minuscule frog on a precision scale is a reminder of the delicate balance involved in caring for creatures great and small.
According to LondonWorld, the weigh-in serves several crucial purposes. First and foremost, it allows keepers to monitor the health of each animal over time. Weight changes can signal everything from impending illness to pregnancy, making this data vital for early intervention. “Our animals here at London Zoo receive world-class care—and a huge part of this is our meticulous planning of their diets,” Simmonds noted. “Our zoo-tritionist ensures every animal is getting the nutrients they need, while our keepers devise ways to feed the animals that spark all of their natural instincts—keeping them fit and challenged.”
Pregnancy detection is another key benefit. Subtle changes in weight, especially when tracked alongside other measurements, can offer clues that a new arrival may be on the way. For endangered species, this information is especially valuable, helping conservationists plan for births and ensure the best possible care for mothers and offspring alike.
But the impact of the weigh-in doesn’t stop at the gates of London Zoo. Each measurement is meticulously logged into the Zoological Information Management System (ZIMS), a global database shared with animal carers and conservationists around the world. This vast repository of information allows experts to compare data on thousands of endangered species, track long-term trends, and collaborate on conservation strategies that span continents. In a world where biodiversity is under constant threat, these shared records are a lifeline for many species.
The spectacle of the weigh-in has also captured the public’s imagination, with adorable footage circulating online of penguins, capybaras, and lemurs taking their turns on the scales. Who could resist the sight of a Humboldt penguin shuffling onto a platform or a ring-tail lemur pausing mid-leap to be measured? These moments offer a rare glimpse behind the scenes and highlight the dedication of the keepers who care for the zoo’s diverse inhabitants.
Of course, not every animal is as cooperative as Priscilla or the capybaras. Some require a bit of gentle persuasion, while others must be outsmarted entirely. Hidden scales, treats, and even a touch of trickery are all part of the zookeepers’ toolkit. The process can be time-consuming and, at times, comical—but the data collected is invaluable.
For the keepers, the weigh-in is also a chance to check in on the animals’ overall well-being. Sudden weight loss or gain can be a red flag, prompting further investigation and, if necessary, adjustments to diet or medical care. In some cases, these routine checks have caught health issues early, allowing for prompt treatment and better outcomes.
Beyond health and conservation, the weigh-in is a logistical marvel. Coordinating the weighing of more than 400 animals—each with its own personality, preferences, and quirks—requires careful planning and teamwork. The zoo’s nutritionist works closely with keepers to ensure that every animal’s diet is tailored to its specific needs, taking into account age, activity level, and any special requirements. Feeding time is more than just a meal; it’s an opportunity to engage animals’ natural instincts, encourage exercise, and provide mental stimulation.
As the weigh-in wrapped up, the data collected was promptly uploaded to ZIMS, where it will be analyzed and shared with colleagues around the globe. For the animals, it was back to business as usual—lounging in the sun, splashing in the water, or exploring their enclosures. For the keepers and conservationists, however, the information gathered represents another step forward in the ongoing effort to protect and preserve the world’s most vulnerable species.
While the event itself may seem lighthearted—filled with cute moments and playful antics—it underscores the serious commitment of London Zoo to animal welfare and global conservation. Each measurement, no matter how small, plays a part in a much larger story: one of science, stewardship, and hope for the future of wildlife everywhere.
As the annual weigh-in becomes a highlight for both staff and the public, it’s clear that these numbers on a scale are more than just data points—they’re a testament to the care, ingenuity, and passion that drive the world’s leading zoos. And for Priscilla, Gizmo, Kiwi, and their fellow residents, it’s just another day in a life well-lived (with maybe a few extra snacks along the way).
